Baffle for perforated electrode in a crossed-field switch device

ABSTRACT

In the nonconducting state, breakdown between concentric electrodes is determined by the Paschen law. If the inner electrode which defines the interelectrode space is perforated, ionization can occur therein, thus reducing holdoff voltage at a given pressure. Path length-limiting shielding adjacent to the electrode perforations prevents ionization within the electrode to maintain holdoff voltage, corresponding to the original electrode spacing.

United States Patent [19 Hofmann [111 3,769,537 1 Oct. 30, 1973 2,786,956 3/1957 Watrous 313/204 3,558,960 1/1971 l-lofmann 313/161 3,638,061 1/1972 Lutz et al 313/161 FOREIGN PATENTS OR APPLICATIONS 700,832 12/1964 Canada 313/204 Primary Examiner-John W. Huckert Assistant ExaminerAndrew J. James Att0rneyW. H. MacAllister, Jr. et a1.

[57] ABSTRACT In the nonconducting state, breakdown between concentric electrodes is determined by the Paschen law. 11 the inner electrode which defines the interelectrode space is perforated, ionization can occur therein, thus reducing holdoff voltage at a given pressure. Path length-limiting shielding adjacent to the electrode perforations prevents ionization within the electrode to maintain holdoff voltage, corresponding to the original electrode spacing.

9 Claims, 5 Drawing Figures DC Sup Load PAIENTEDumaonsn 3.769.537

sum 2 or 2 Fig. 4.

V Fig. 5.

BAFFLE FOR PERFORATED ELECTRODE IN A CROSSED-FIELD SWITCH DEVICE BACKGROUND This invention is directed to a crossed field switch device, and particularly to path length-limiting baffling to control electron path length for control of the conditions under which cascading ionization will take place.

In one class of crossed field switch devices of the prior art, a circular or annular interelectrode space is provided. When an electric field is radially applied and a magnetic field is axially applied, the electron path is substantially circular through the annulus, and the electron path length in conjunction with the gas in the interelectrode space provides for cascading ionization, which in turn permits interelectrode electric conduction. In the absence of a magnetic field, the electron path is radial and is too short for the cascading ionization. When one or both of the electrodes is perforated, in order to supply gas to the interelectrode space as it is depleted, the perforations permit some of the electrons to pass on a longer path, even in the absence of a magnetic field, thus reducing the Paschen holdoff voltage by effectively increasing the interelectrode spacing. This invention is an improvement on the structure shown in G. A. G. Hofmann and R. C. Knechtli U.S. Pat. No. 3,558,960, the entire disclosure of which is incorporated herein by this reference.

SUMMARY In order to aid in the understanding of this invention,

directed to a crossed field switch device which has at least one perforated electrode which defines the interelectrode space, and has a baffle in association with the perforations to limit the maximum electron path length in the absence of a magnetic field.

Accordingly, it is an object of this invention to provide a crossed field switch device which has improved precision of the conditions which define off and on switching. It is yet another object to provide a crossed field switch device wherein at least one electrode is perforated to provide replenishment of gas supply in the interelectrode space as the interelectrodegas is depleted, and has a baffle in association with the perforations to limit the electron path length.

Other objects and advantages of this invention will become apparent from the study of the following portion of the specification, the claims, and the attached drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view, with parts broken away and parts shown in section, of a crossed field switch device constructed in accordance with this invention.

FIG. 2 is an enlarged perspective view ofa portion of the baffle construction employed in conjunction with the perforated electrode in the crossed field switch device.

FIG. 3 is a section taken generally along the line 33 of FIG. 2.

FIG. 4 is a transverse section through the crossed field switch device of FIG. 1.

FIG. 5 is a Paschen curve showing the conditions for conductivity in the interelectrode space.

DESCRIPTION The crossed field switch device of this invention is generally indicated at 10. It is serially connected with a DC electric supply 12 and a load 14. On and offswitching of the crossed field switch device 10 thus controls the flow of electric current from supply 12 through load 14.

Further referring to FIG. 1, crossed field switch device 10 comprises housing 16 which is carried upon bottom flange 18. Bottom flange 18 is, in turn, mounted upon base flange 20. The flanges are secured together, as by conventional nuts and bolts, to provide a tight seal. Base flange 20 stands upon foot 22 for supporting the crossed field switch device structure. F urthermore, a vacuum connection can be connected to the bottom of foot 22 for controlling the pressure on the interior of housing 16 and controlling the type of gas within the housing. Hydrogen, including its isotope, is a satisfactory gas. Housing 16, together with its base flange 20, serves as a suitable vacuum-tight envelope.

Cathode 24 is in the form of a cylindrical tube. It is spaced inwardly from housing 16. The cathode 24 has a lower end closure 26. Stand-off 28 supports cathode 24 from lower flange 20. Lower end closure 26 does not need to effect closure, but simply provides mechanical support for the cathode and reduces plasma end losses. By this construction, the entire cathode can be withdrawn downwardly through the large opening in bottom flange 18, when flanges 18 and 20 are separated. By this means, inspection and service of the cathode, as well as inspection and service of the interior of housing 16, are accomplished. Cathode 24 is metallic and can be made of stainless steel. Cathode 24 is electrically connected through its lower end closure 26 and stand-off 28 to flange 20. Therefore, electrical connection to the cathode can be made either directly through flange 20 or through foot 22. Cathode 24 preferably has an axial slot to prevent the circumferential circulation of current during switching transients, when the axial magnetic field changes with time.

Anode 30 is of cylindrical tubular construction and is positioned concentrically within cathode 24 to provide a radial space 32. The radial space 32 is substantially equal at all facing positions of the anode and cathode. Housing 16 has a top cap 34 of electrically insulative material. Anode 30 is carried on top end closure 36 which has a central mounting stud 38. Central mounting stud 38 extends through top cap 34 in vacuum-tight connection to provide an electrical connectionthrough the anode, as well as structural mounting for the anode.

Anode 30 has aplurality of holes or perforations 40 therethrough so that the interior space within hollow anode 30 is in communication with the interelectrode space 32. The volume within the interior of anode 30 is thus in gas communication with the interelectrode space. Magnet 42 is positioned on the exterior of housing 30 in such a manner as to provide magnetic lines of force in the interelectrode space 32 which are substantially parallel to the axis of the electrodes, at least over a substantial part of the electrode length. Magnet 42 is illustrated as being an electromagnet, and such is preferred so that the magnetic field can be readily switched on and off. The power supply to magnet 42 is preferably of such nature as to provide for rapid turnon and off of the field. Its strength is such as to provide a field between 25 and 150 gauss. Seventy gauss was found to be a preferred value, Considering the turnon and turnoff effects, as well as magnet power consumption.

The interior of anode 30, as well as the interelectrode space, is filled with a gas to an appropriate pressure. Referring to FIG. 5, the Paschen curve is shown therein. This curve illustrates conditions of conductivity in glow discharge through a gas. The area above the curve of FIG. is a conductive region, while the area below and to the left, as well as below and to the right of the curve line is a nonconductive region. The voltage V is the voltage applied to the interelectrode space. It is the electric field applied to the space. The value of p is the gas pressure in the interelectrode space, while the value of d is the electron path length. When there is no magnetic field, d is equal to the interelectrode radial space.

When the magnetic field is off, electron flow is only under the influence of the electric field from the cathode to the anode so that the average electron path length is substantially equal to the interelectrode space d, and is less than the electron mean-free path length. Thus, there is no sustained ionization, electron flow is low, and the switching device can withstand a high standoff voltage. Its operating point lies below and to the left of the curve. When the magnetic field is applied to the interelectrode space by electromagnet 42, the axial magnetic field causes the electron path to spiral through the interelectrode annulus until a collision occurs. In this longer path caused by the magnetic field effect, there are sufficient collisions to maintain cascading ionization. In this case, the product pd, where d is the electron path length, is in the conductive region of the Paschen curve. Thus, so long as a sufficient magnetic field is applied, once electrons start flowing, the flow is maintained until the magnetic field is cut off. When cut off, the electrons again flow radially so that ionization is not maintained.

Since the net electron flow is from the cathode to the anode, and flow of electrons through the interelectrode space results in collisions with gas atoms to cause ionization, a certain number of these ionizing collisions cause the gas ionsto be driven into the surface of the cathode. Gas pumping by ion implantation and by adsorption on freshly-sputtered material occurs with the result that the amount of ionized and neutral gas decreases after the switching device has been conducting for a period of time. As the gas decreases to a certain low point, the product pd moves out of the conductive region by this decrease in pressure. Thus, conduction is cut off. In order to maintain this time as long as possible, the interior space of anode 30 is employed as a gas volume, and the gas is permitted to flow out through anode holes 40 into the interelectrode space 32.

In order to control the electron path length, in order to limit the electron path length so that there is not an extended path length d through the holes 40, baffle 44, see FIGS. 2 and 3, is provided. Baffle 44 can be any type of construction which permits gas passage'from the interior of the anode out through holes 40 in the anode into the interelectrode space 32. However, baffle 44 should limit the line-of-sight through the holes 40. As a particular preferred embodiment of baffle 44, a plurality of supported rings is illustrated in FIGS. 1, 2, 3, and 4. A plurality of axially spaced rings, two of which are indicated at 46 and 48, are positioned on the interior of anode 30. They are of sufficient diameter that there is an annular space around the outside of these outer rings and interiorly of anode 32, see FIG. 4. The rings are secured to posts standing through the interior thereof. Four posts 50., 52, 54, and 56 are illustrated. As indicated in FIGS. 1, 2, and 3, the outer rings are axially spaced along their mounting posts. For example, space 58 is seen between outer rings 46 and 48. In order to obstruct this space to line-of-sight flight of electrons, inside rings are provided to overlap the space. In FIG. 3, inside rings 60 and 62 are shown as being axially spaced and axially positioned to overlap the spaces between the outer rings. The inside rings are positioned interiorly of posts 50, 52, 54.,and 56 and are secured thereto. The entire structure of baffle 44 is preferably of metallic material and is supported from the anode, at least at top end closure 36. Thus, the baffie 44 is at anode potential. The baffle thus limits the path length of electrons flowing in a radial direction, so that the product pd can be maintained low in the absence of a magnetic field. As seen in FIG. 5, this provides a high holdoff voltage.

In-considering the physical structure, either or both the inner or outer electrodes can be perforated. If the outer electrode is perforated so that the space between the outer electrode and the housing can also contribute gas to the interelectrode space, it is desirable to use baffles adjacent those perforations to also limit the straight line paths through the perforation. Similarly, in the structure shown in FIG. 1, the anode and the cathode can be reversed simply by reversing the potential. However, it is desirable to maintain the cathode area as large as possible. For this reason, the cathode is unperforated and is placed as the exterior electrode. In addition to limiting the straight line distance through the anode perforations, the switch device is constructed so that, in the end chambers at the ends of the electrodes, straight line electron paths are limited by close placement of adjacent structures or by insertion of floating electrodes or baffles.

This invention having been described in its preferred embodiment, it is clear that it is susceptible to numerous modifications and embodiments within the ability of those skilled in the art and without the exercise of the inventive faculty. Accordingly, the scope of this invention is defined by the scope of the following claims.

What is claimed is:

1. A crossed field switch device comprising:

first and second spaced electrodes defining an interelectrode space therebetween, gas in said interelectrode space and means for controllably applying a magnetic field to said interelectrode space so that, when an electric field is applied between said electrodes, glow discharge of electrode current conduction takes place in the interelectrode space between the electrodes when the magnetic field is above a critical value and does not take place when the magnetic field is below the critical value, said second electrode being perforated so that gas can move through the electrode perforations into the interelectrode space to aid in maintaing glow discharge, the improvement comprising:

a baffle positioned adjacent said second electrode on the side of said second electrode away from said interelectrode space so as to not interfere with the glow discharge in the interelectrodespace and positioned adjacent said second electrode perforations to limit the length of the electron flow path, from the interelectrode space through said perforations.

2. The crossed field switch device of claim 1 wherein:

said first and second electrodes are tubular and said interelectrode space is an annular space, said means for applying a magnetic field in the annular interelectrode space providing a field which is substantially parallel to the tubular axis.

3. The crossed field switch device of claim 2 wherein said first electrode is the outer tubular electrode and said second electrode is the inner tubular electrode, said first electrode being a cathode and said second electrode being an anode.

4. The crossed field switch device of claim 3 wherein said anode electrode is said perforated electrode.

5. The crossed field switch device of claim 4 wherein said baffle comprises spaced rings positioned inside said perforated anode electrode. 7

6. The crossed field switch device of claim 5 wherein said spaced rings are mounted upon posts extending substantially parallel to said axis.

7. The crossed field switch device of claim 6 wherein inner rings are positioned interiorly of said posts and mounted on said posts and are positioned to overlap the spaces between said spaced electrodes.

8. The crossed field switch device of claim 7 further including a supply of electric potential and an electric load serially connected with said crossed field switch device so that offswitching of said crossed field switch device cuts off current flow from said supply through said electric load.

9. The method of operating a crossed field switch device which has a tubular cathode electrode and a perforated tubular anode electrode spaced concentrically therein and which has gas at reduced pressure in the interelectrode space and within the tubular anode so that electric current conduction by glow discharge occurs between the electrodes in the presence of an electric field and an axial-magnetic field in the interelectrode space, comprising the step of:

limiting electron flow into the interior space of the tubular anode through the anode perforations by positioning a baffle behind the perforations away from the interelectrode space to obstruct straight line electron passage through the anode perforations into the interior space of the anode electrode. 

1. A crossed field switch device comprising: first and second spaced electrodes defining an interelectrode space therebetween, gas in said interelectrode space and means for controllably applying a magnetic field to said interelectrode space so that, when an electric field is applied between said electrodes, glow discharge of electrode current conduction takes place in the interelectrode space between the electrodes when the magnetic field is above a critical value and does not take place when the magnetic field is below the critical value, said second electrode being perforated so that gas can move through the electrode perforations into the interelectrode space to aid in maintaing glow discharge, the improvement comprising: a baffle positioned adjacent said second electrode on the side of said second electrode away from said interelectrode space so as to not interfere with the glow discharge in the interelectrode space and positioned adjacent said second electrode perforations to limit the length of the electron flow path from the interelectrode space through said perforations.
 2. The crossed field switch device of claim 1 wherein: said first and second electrodes are tubular and said interelectrode space is an annular space, said means for applying a magnetic field in the annular interelectrode space providing a field which is substantially parallel to the tubular axis.
 3. The crossed field switch device of claim 2 wherein said first electrode is the outer tubular electrode and said second electrode is the inner tubular electrode, said first electrode being a cathode and said second electrode being an anode.
 4. The crossed field switch device of claim 3 wherein said anode electrode is said perforated electrode.
 5. The crossed field switch device of claim 4 wherein said baffle comprises spaced rings positioned inside said perforated anode electrode.
 6. The crossed field switch device of claim 5 wherein said spaced rings are mounted upon posts extending substantially parallel to said axis.
 7. The crossed field switch device of claim 6 wherein inner rings are positioned interiorly of said posts and mounted on Said posts and are positioned to overlap the spaces between said spaced electrodes.
 8. The crossed field switch device of claim 7 further including a supply of electric potential and an electric load serially connected with said crossed field switch device so that offswitching of said crossed field switch device cuts off current flow from said supply through said electric load.
 9. The method of operating a crossed field switch device which has a tubular cathode electrode and a perforated tubular anode electrode spaced concentrically therein and which has gas at reduced pressure in the interelectrode space and within the tubular anode so that electric current conduction by glow discharge occurs between the electrodes in the presence of an electric field and an axial-magnetic field in the interelectrode space, comprising the step of: limiting electron flow into the interior space of the tubular anode through the anode perforations by positioning a baffle behind the perforations away from the interelectrode space to obstruct straight line electron passage through the anode perforations into the interior space of the anode electrode. 